As information devices such as personal computers and hard disc recorders have come to achieve high function and high speed, amounts of information handled by users have accordingly more and more increased in recent years. Therefore, storage media for information recording devices are demanded to have higher recording density. To increase recording density, it is necessary to miniaturize the size of each recording cell or recording mark which is a recording writing unit in recording media. However, miniaturization of recording cells or marks in conventional storage media has encountered great difficulties.
Storage media for existing hard disc devices have a structure in which a granular thin film having a thickness of 10 nm or so is deposited on a disc substrate. If grains of the granular thin film are formed to be small to improve recording density, polycrystal causes instable recording due to thermal fluctuation (specifically, as a volume of magnetic grains decreases, a ratio of magnetic energy to thermal energy decreases, and magnetization in recording varies or ceases under influence of temperature). Therefore, recording becomes instable or noise increases when recording cells are small, while no problem occurs when recording cells are large. This is caused by decrease in number of crystal grains included in recording cells and by relative increase of interaction between recording cells each other.
To avoid this problem, as next-generation magnetic storage media which will take over thin-film media, there have been proposed bit-patterned media (BPM) in which a recording material is divided in advance by a non-recording material and recording/reproducing is performed, considering one single grain of the recording material as one unit recording cell (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2007-73116 (paragraphs 0024 to 0027), Jpn. Pat. Appln. KOKAI Publication No. 2007-305289 (paragraphs 0012 to 0020), and Jpn. Pat. Appln. KOKAI Publication No. 2003-151103 (paragraphs 0025 to 0026)).
The bit-patterned media each are configured to employ a magnetic dot array in which nanometer-scale micro magnets (magnetic dots) are regularly arrayed on a substrate. Digital signals “0” and “1” (where each one dot corresponds to one bit) are recorded as magnetization directions of each magnetic dot. In the bit-patterned media, each bit is absolutely physically independent, and therefore, noise is not caused in principle by magnetization transition as a major factor which hinders improvement to attain higher recording density in continuous film media.
On the other side, a writing position may shift off in a track direction because a timing to start writing shifts when a recording head records data at a particular position on a storage medium for existing hard disc devices in which a granular thin film is deposited. Even then, write errors hardly occur because the medium has a uniform surface.
However, in bit-patterned media in which a recording material is divided by a non-recording material on a surface of a storage medium, matching of a timing to start recording is significant because it is necessary to write data into every one of divided recording cells when a recording head records data at a particular position on a storage medium. If the timing to start writing shifts off, the recording head performs a writing operation, spreading over a part of a non-recording material or an adjacent recording cell. Therefore, write errors increase.
Array patterns for the bit-patterned media may be grid (square) patterns and staggered patterns. In a grid pattern in which dots are arrayed vertically and horizontally as disclosed in the foregoing Publication No. 2007-73116, each one dot row is considered as one track, and therefore, accuracy is required for constraint conditions concerning a cross-track direction, such as a head core width and tracking.
As solutions thereof, staggered patterns as described in the foregoing Publications No. 2007-305289 and No. 2003-151103 have been discussed. In such staggered patterns, a large number of dot rows are arrayed at a constant dot pitch, and odd-numbered dot rows thereof and even-numbered dot rows thereof are shifted from each other by 180 degrees in phase. A head has a width capable of simultaneously accessing two dot rows. During recording, writing is performed in an order of a first dot in a first row, a first dot in a second row, a second dot in the first row, . . . , while moving the head along the dot rows. Similarly, during reproduction, reading is performed in an order of the first dot in the first row, the first dot in the second row, the second dot in the first row, . . . , while moving a head along the dot rows. Thus, adjacent two dot rows are used as one data track. Accordingly, the staggered patterns have a double track pitch and a half bit pitch, compared with the grid patterns. A factor which decides a write phase margin is a space between dots (a dot pitch). However, the staggered patterns use two dot rows as one data track, and therefore, have a half dot pitch. Consequently, the staggered patterns have a smaller write phase margin than the grid patterns. Therefore, stricter accuracy is required for manufacturing discs and heads for the staggered patterns.
Thus, in magnetic recording devices using conventional bit-patterned media, the write phase margin is narrow and therefore causes a drawback of easy occurrence of write errors. To prevent the drawback, higher processing accuracy in manufacture is required strictly.